BIPOLAR MOLECULAR OUTFLOWS FROM YOUNG STARS AND PROTOSTARS

A violent outflow of high-velocity gas is one of the first manifestati ons of the formation of a new star. Such outflows emerge bipolarly fro m the young object and involve amounts of energy similar to those invo lved in accretion processes. The youngest (proto-)stellar low-mass obj ects known to date (the Class 0 protostars) present a particularly eff icient outflow activity, indicating that outflow and infall motions ha ppen simultaneously and are closely linked since the very first stages of the star formation processes. This article reviews the wealth of i nformation being provided by large millimeter-wave telescopes and inte rferometers on the small-scale structure of molecular outflows, as wel l as the most recent theories about their origin. The observations of highly collimated CO outflows, extremely high velocity (EHV) flows, an d molecular ''bullets'' are examined in detail, since they provide key information on the origin and propagation of outflows. The peculiar c hemistry operating in the associated shocked molecular regions is disc ussed, highlighting the recent high-sensitivity observations of low-lu minosity sources. The classification schemes and the properties of the driving sources of bipolar outflows are summarized with special atten tion devoted to the recently identified Class 0 protostars. All these issues are crucial for building a unified theory on the mass-loss phen omena in young stars.